The present invention relates in general to touch screen displays, and, more specifically, to touch screens employing a cross-point matrix with paired light emitters/detectors disposed around the periphery of a display area which are scanned in order to detect blockage by a finger, stylus, or other pointing device pressed against the touch screen display.
Infrared (IR) touch screens are widely used to correlate finger touches on a display screen by a user with soft buttons or soft keys displayed at various locations on the display screen. Typically, a number of IR emitters (i.e., transmitters) and detectors (i.e., receivers) are arranged around the periphery of the display screen to create a plurality of intersecting light paths. When the user touches the display, his or her finger blocks the IR transmission of certain ones of the perpendicularly arranged transmitter/receiver pairs. Based on the identity of the blocked pairs, the touch screen system can determine the location of the intercept.
IR light emitting diodes (LED's) and IR phototransistors are typically used for the emitters and detectors, respectively, and are mounted on a ring-shaped circuit board that is affixed to the front of the display screen. The circuit density of the LED/phototransistor portion of a cross-point matrix touch screen is proportional to the number of LED/phototransistor pairs deployed in the matrix (which in turn is a function of the number of touch-point locations to be sensed) and the size of the ring board. Because the light emitted from each LED cannot easily be constrained to a narrow path directed at only one phototransistor, the pairs must be activated sequentially. The multiplexing of the LED/phototransistor pairs has necessitated the use of a large number of conductor traces on the ring board to bring individual power and/or control signals to each pair and to bring out the detected output signals.
Since the ring board must be positioned outside of the display area, the overall package size of the front of a touch-screen unit has to be larger than the size of the usable display area. A large border around the display area may be undesirable in many applications, such as in automotive dashboard displays where space is limited. The border also places limits on styling of a unit and the ability to put other components (e.g., mechanical control knobs or switches) close to the display area. Furthermore, a larger size results in higher costs for the ring board and the unit as a whole.